This invention relates to new and improved fluorescent intensifying screens (or radiographic phosphor panels) used in imaging from X-radiation. In particular, it relates to such screens having multilayer reflective polymeric supports that exhibit a different level of reflectance of light depending u on the angle of light incidence. The invention also relates to imaging assemblies containing such screens.
In silver halide photography one or more radiation sensitive emulsion layers are coated on a support and image-wise exposed to electromagnetic radiation to produce a latent image in the silver halide emulsion layer(s). The latent image is converted to a viewable image upon subsequent chemical photoprocessing.
Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element to X-rays. In 1913 the Eastman Kodak Company introduced its first silver halide photographic element specifically intended to be exposed by X-radiation (that is, its first silver halide radiographic element).
The medical diagnostic value of radiographic imaging is widely accepted. Nevertheless, the desirability of limiting patient exposure to X-radiation has been appreciated from the inception of medical radiography. Silver halide radiographic elements are more responsive to longer wavelength electromagnetic radiation than to X-radiation.
Low X-radiation absorption by silver halide radiographic elements as compared to absorption of longer wavelength electromagnetic radiation led quickly to the use of fluorescent intensifying screens (hereinafter, radiographic phosphor panels) when the Patterson Screen Company in 1918 introduced matched intensifying screens for Kodak""s first dual coated radiographic element.
A radiographic phosphor panel contains on a support a fluorescent phosphor layer that absorbs X-radiation and emits longer wavelength radiation to an adjacent radiographic element in an imagewise pattern corresponding to that of the X-radiation received.
The need to increase the diagnostic capabilities of radiographic imaging while minimizing patient exposure to X-radiation has presented a significant, long-standing challenge in the construction of both radiographic elements and intensifying screens. In constructing radiographic phosphor panels, the goal is to achieve the maximum longer wavelength electromagnetic radiation emission possible for a given level of X-radiation exposure (which is realized as maximum imaging speed) while obtaining the highest achievable level of image definition (that is, sharpness or acuity). Since maximum speed and maximum sharpness in radiographic phosphor panel construction are not compatible features, most commercial panels represent the best attainable compromise for their intended use.
The choice of a support for a radiographic phosphor panel illustrates the mutually exclusive choices that are considered in panel optimization. It is generally recognized that supports having a high level of absorption of emitted longer wavelength electromagnetic radiation produce the sharpest radiographic images. Radiographic phosphor panels that produce the sharpest images are commonly constructed with black supports or supports loaded with carbon particles. Often transparent panel supports are employed with the panel being mounted in a cassette for exposure along with an absorbing backing layer. In these constructions, sharpness is improved at the expense of photographic speed because a portion of the otherwise available, emitted longer wavelength radiation is not directed to the adjacent radiographic element.
If a black or transparent radiographic phosphor panel support is replaced by a more reflective support, a substantial increase in speed can be realized. The most common conventional approach is to load or coat a screen support with a white pigment such as titania or barium sulfate. U.S. Pat. No. 3,787,238 (Juliano), U.S. Pat. No. 4,318,001 (Degenhardt), and U.S. Pat. No. 4,501,971 (Ochiai) are illustrative of the use of such supports.
Thus, conventional supports for radiographic phosphor panels include cardboard, plastic films such as those of cellulose acetate, polystyrene, and poly(methyl methacrylate). Particularly preferred are films of poly(ethylene terephthalate). The plastic supporting films may contain light absorbing materials such as dyes or pigments such as carbon black, or may contain light reflecting materials such as titanium dioxide or barium sulfate. The light absorbing materials may be appropriate in situations where increased definition is required while the light reflecting materials are used to enhance panel speed.
However, even the best reflective supports known in the art have degraded image sharpness in relation to imaging speed so as to restrict their use to situations wherein image definition (or sharpness) is less demanding. Further, many types of reflective supports that have been found suitable for other purposes have been tried and rejected for use in fluorescent intensifying screens. For example, the loading of the supports with optical brighteners, widely employed as xe2x80x9cwhitenersxe2x80x9d, has been found to be incompatible with achieving satisfactory image sharpness.
By a process of trial and error over a development period of approximately 70 years the radiographic phosphor panel art has developed a preference for reflective supports from a relatively limited class of constructions. In addition, workers in the art have generally not chosen supports that, though nominally reflective, were developed for other, less demanding purposes.
During the last 25 years as the potentially deleterious effects of even low levels of X-radiation exposure have been publicly called into question, every obvious improvement and continual innovation have increased the capabilities of diagnostic radiographic imaging while reducing patient X-ray exposure.
There exists in the art a class of reflective supports hereinafter referred to as xe2x80x9cstretch cavitation microvoidedxe2x80x9d supports. For example, U.S. Pat. No. 3,154,461 (Johnson) discloses a polymeric film loaded with microbeads of calcium carbonate of from 1 to 5 xcexcm in size. By biaxially stretching the support, stretch cavitation microvoids were introduced, rendering the support opaque.
Primary interest in stretch cavitation microvoided supports has centered on imparting to polymer film supports paper-like qualities, as illustrated in U.S. Pat. No. 4,318,950 (Takaski et al.), U.S. Pat. No. 4,340,639 (Toyoda et al.), U.S. Pat. No. 4,377,616 (Ashcraft et al.), U.S. Pat. No. 4,438,175 (Ashcraft et al.), and H. H. Morris et al., xe2x80x9cWhite Opaque Plastic Film and Fiber for Papermaking Use,xe2x80x9d ACS Div. Org. Coatings Plastic Chemistry, Vol. 34, pp.75-80, 1974.
More recently, stretch cavitation microvoided supports have been considered as possible replacements for photographic print supports, as illustrated in U.S. Pat. No. 3,944,699 (Matthew et al.), U.S. Pat. No. 4,187,113 (Matthews et al.) and U.K. Pat. No. Specifications 1,593,591 and 1,593,592 (both Remmington et al.). Polypropylene microbeads are in one instance employed, but the preferred microbeads are white pigment barium sulfate microbeads. U.S. Pat. No. 4,912,333 (Roberts et al.) proposes the use of reflective lenslets.
Other stretch cavitation microvoided shaped articles, such as films, sheets, bottles, tubes, fibers, and rods, are also known wherein the polymer forming the continuous phase is a polyester and the microbeads are composed of a cellulose ester.
U.S. Pat. No. 4,912,333 (Roberts et al.) suggests that stretch cavitation microvoided supports might be suitable for the demanding properties needed in radiographic phosphor panels.
U.S. Pat. No. 6,027,810 (Dahlquist et al.) discloses improved radiographic phosphor panel performance with the use of an antistatic material in a top protective layer or in the phosphor layer.
U.S. Pat. No. 5,475,229 (Itabashi et al.) discloses a novel radiographic phosphor panel that has improved durability with the uses of thermoplastic binder and in particular a fluoro-resin coated over the phosphor layer.
The use of reflective bases to enhance screen speed is well known in the art, and many current screens (KODAK LANEX Regular) for example are coated on titanium dioxide or other white bases to provide a speed advantage. Typically, reflection is obtained from a portion of the volume of the support. That is, the reflectance is not only from the surface of the support, but extends some distance into the support. These layers provide increased speed in proportion to their reflectance, but with each increment in speed gained, there is a loss in sharpness due to the diffuse nature of the reflectance, both in the screen and in the reflective support.
Specular reflectors such as those formed from evaporated metal films (aluminum, nickel, etc.) can also be used as panel supports. The common specular reflectors have disadvantages however in that they generally have lower maximum reflectance than the diffuse reflectors at the wavelengths of light emitted by the common phosphor materials. Moreover, the evaporated metal layers are relatively fragile and if a phosphor layer is coated directly onto the reflective metal layer, substantial reflectance is lost. A polymeric film can be applied to the metal reflector to protect it from the coating solvents, but the presence of this film separates the reflector from the phosphor layer and can cause flare light that is damaging to the resulting image.
U.S. Pat. No. 5,795,708 (Boutet) describes the use of a dichroic mirror antihalation layer to increase speed and improve sharpness for heat processable films. The continuous dichroic mirror layer is formed from multiple alternating layers of silicon dioxide and titanium dioxide and is coated on top of a base layer (that is a support).
Multilayer polymeric stacks have also been disclosed that function as wavelength selective reflectors such as xe2x80x9ccold mirrorsxe2x80x9d that reflect visible light but transmit infrared or xe2x80x9chot mirrorsxe2x80x9d that transmit visible and reflect infrared. Examples of a wide variety of multilayer stacks are included in U.S. Pat. No. 5,882,774 (Jonza et al.).
Copending and commonly assigned U.S. Ser. No. 09/883,801 (filed Jun. 18, 2001 by Aylward, Steklenski, and Elman) describes and claims the use of polymeric multi-layer reflectors in radiographic phosphor panels.
There is a need in the art for radiographic phosphor panels that have increased photographic speed without a loss in image sharpness. There is a need for such panels to be designed using a specular reflector with high reflectance and robustness to coating solvents. There is also a need to obtain such high reflectance to maximize speed gain (reduced patient dose) while the specular nature of the reflector and its location directly under the phosphor layer would minimize sharpness losses. The resulting improvement in the speed and sharpness of the panel would provide broader latitude in the design of pairs of panels for improved diagnostic capability, especially in mammography.
The present invention provides a radiographic phosphor panel comprising a phosphor layer adjacent to a polymeric multi-layer reflector that exhibits a different level of reflectance of light depending upon the angle of light incidence that is greater than 0 and up to and including 90 degrees, and on the opposing side of the reflector, a light absorbing substrate.
Still further, this invention provides a radiographic imaging assembly comprising at least one radiographic phosphor panel described above that is arranged in association with a photosensitive recording material, such as a silver halide radiographic film.
The present invention provides a number of advantages. It provides a radiographic phosphor panel (or fluorescent intensifying screen) that provides both increased photographic speed and sharpness in imaged photosensitive recording materials.
These and other advantages of the invention are accomplished by the use of specific polymeric multi-layer reflectors (generally as the support) in the radiographic phosphor panel with a phosphor layer on one side and a light absorbing substrate on the opposite side of the support.
However, unlike the invention described in U.S. Ser. No. 09/883,801, noted above, not just any polymeric multi-layer reflector can be used in the practice of this invention. The required polymeric multi-layer reflectors exhibit a different level of reflectance of light (that is different % reflectance of light) depending upon the angle of light incidence of between 0 and up to and including 90 degrees to the plane of the reflector. Thus, an essential feature of the present invention is the use of polymeric multi-layer reflectors that reflect most light striking them at xe2x80x9chigh incident anglesxe2x80x9d and transmit and absorb most light striking them at xe2x80x9clow incident anglesxe2x80x9d. This low incident angle light is generally absorbed by the light absorbing substrate on the opposite side of the reflector from the phosphor layer in order to provide optimal function of the panel.
For example, in one embodiment of this invention, a polymeric multi-layer reflector can be designed to reflect as much as 90% of the light striking it between 45 and 90 degrees while transmitting a substantial portion of the light striking it at less than 45 degrees angle of incidence. The high degree of reflectance at the higher angles of incidence provide excellent photographic speed while the higher transmittance of light at the lower angles of incidence reduces scatter and improves image sharpness.